Molecular Breast Imaging (MBI) is a test that uses a special gamma camera to detect breast cancer. Rather than simply taking a picture of the breast, molecular breast imaging is a type of functional imaging. This means that the pictures it creates reflect differences in the behavior of cells rather than just the appearance, so it can find tumors that may look the same as background tissue on a mammogram. Brightness on the image correlates with how rapidly the cells that are visualized are growing or dividing. The test is done by injecting a small amount of a radioactive tracer into a vein, the same radiotracer that has been used for many years in cardiac stress testing, and the pattern of tracer uptake in breast cells is then detected by the gamma detector.
This is a relatively new technique that may be particularly helpful in screening for breast cancer in women with dense breast tissue. Half of U.S. states have adopted legislation mandating that women who undergo mammography be informed if they have dense breasts. This is because there is a risk that mammographically dense breast tissue may mask a cancer. Women who receive this notification are advised to talk to their doctor about additional options for screening, but there is not agreement among doctors regarding which additional screening options are best.
MBI is one option for improving cancer detection in women with dense breasts. Although it is available at some centers, it is not yet widely available. In addition to its role in screening for breast cancer, MBI can be used to evaluate a questionable area on another breast imaging test.
MBI is viewed as a safe procedure. The amount of radiation delivered is well within the safe limit for humans and similar to that from other commonly used radiology tests. For most women, the risk of radiation is outweighed by the potential benefits of the test. Allergic reactions to the tracer are extremely rare. If it is abnormal, MBI may lead to additional testing or biopsies. The test will not detect all cancers. Some cancers may be too small to permit detection, too slow growing to permit detection, or they may be located in a part of the breast that is not well visualized by the study. MBI is performed by having the patient sit in a chair facing the system which looks similar to a mammogram machine. One breast at a time will be placed on the flat surface of a gamma camera and a second camera will be lowered on the top of the breast. The degree of compression that is used for this test is much less than is used for standard mammograms and should not be uncomfortable. Each breast will be imaged for approximately 10 minutes. A radiologist will interpret the test. If a high concentration of the tracer is seen in one area, the doctor then may recommend additional testing and perhaps an image-guided biopsy for further evaluation. Recent studies have shown that in women with dense breasts who present for routine screening, the addition of MBI to mammography increases the number of cancers detected from 3 per 1000 women screened to 9 per 1000. This has made MBI a compelling alternative to ultrasound and/or MRI as a supplementary screening tool, particularly for women with dense breasts. Compared to ultrasound, MBI has a lower recall rate for additional testing. Compared to MRI, MBI is a quicker test, far less expensive, better tolerated by patients and easier for radiologists to interpret. In the future, a cost effective and safe way for screening may be to alternate mammography with or without tomosynthesis (3-D mammography) one year and MBI alone the following year.